secondary idx in teradata

hi
what is the purpose of secondary indexes in teradata...
most of the articles and books i read, it says secondary indexes adds
overhead in processing..
so,why do we even need the concept of secondary index in teradata
methodolgy...
or am i missing something here?
--
Thanks
Abi

*
A secondary index is an alternate path to the data. Secondary Indexes are
used to improve performance by allowing the user to avoid scanning the
entire table. A Secondary Index is like a Primary Index in that it allows
the user to locate rows. It is unlike a Primary Index in that it has no
influence on the way rows are distributed among AMPs. A database designer
typically chooses a secondary index because it provides faster set
selection.
Primary Indexes requests require the services on only one AMP to access
rows, while secondary indexes require at least two and possibly all AMPs,
depending on the index and the type of operation. A secondary index search
will typically be less expensive than a full table scan.
Secondary indexes add overhead to the table, both in terms of disk
space and maintenance; however they may be dropped when not
needed, and recreated whenever they would be helpful.
**
*
*~Prashanth
*

A secondary index is an alternate path to the data. Secondary Indexes are used to improve performance by allowing the user to avoid scanning the entire table.A secondary index search will typically be less expensive than a full table scan.so,secondary index are chosen to improve table performance.
Also,Secondary indexes add overhead to the table, both in terms of disk space and maintenance,however they may be dropped when not needed, and recreated whenever they would be helpful.